The past decade has seen so many incredible advances in astronomy that it would be hard to single out one as standing above the others. But near the top of my list is the work by Andrea Ghez (UCLA) and her colleagues to measure the mass of the Milky Way’s central black hole. In a paper posted on the Web last week, they derive a new and improved mass for our galaxy's monster in the middle.

The team’s pure measurement yields a mass of 4.1 million Suns with an uncertainty of only 0.6 million Suns. But if the black hole is assumed to be stationary with respect to the rest of the galaxy (and there's no evidence otherwise), its mass rises a bit to 4.5 million Suns with an uncertainty of only 0.4 million. This latter result is significantly higher than previous measurements made by Ghez’s team and by a European group led by Reinhard Genzel (Max Planck Institute for Extraterrestrial Physics, Germany).

A black hole of that mass has a diameter of 0.1 astronomical unit (about 9 million miles).

Why is this measurement so amazing? The center of the Milky Way is 27,000 light-years away, and it's hidden behind thick clouds of gas and dust along our line of sight. How do you measure the mass of an invisible object tens of thousands of light-years from Earth when even its surroundings are obscured from view?

Ghez and her team had to employ all the resources of modern astronomy: a really big telescope, detectors that operate in the infrared, and the relatively recent technology of adaptive optics (AO for short). Oh yeah, they also needed a lot of patience.

For the past decade, they have been observing the galactic center with the 10-meter Keck I and II telescopes in Hawaii. Keck gives them the brute-force light-gathering power to see stars in the galactic center. They observe at infrared wavelengths, which can penetrate the thick clouds of gas and dust. And most of all, they use AO, which involves a laser-generated artificial guide star and flexible, deformable mirrors to compensate for the rapid-fire blurring effects of Earth’s atmosphere.

The combination of these techniques allowed the group to resolve dozens of individual stars near the galactic center. Incredibly, the team could trace the curving orbital motions of several of these stars over more than a decade, and actually create a movie of these motions. Astronomers of just 25 years ago would have considered this magic.

The high mass of the Milky Way’s black hole, known as Sagittarius A* (pronounced "A-star"), made this possible. Anything orbiting near such a massive object is going to move really, really fast. These stars are whirling around the black hole at speeds exceeding 4,500 km per second (10 million miles per hour). One star in particular, dubbed S0-2, has been clocked at nearly 8,000 km/sec. By using simple orbital laws dating back to Isaac Newton in the 1600s, Ghez could use these stellar velocities to derive the mass of the central gravitating object.

In their new paper (accepted for publication in the Astrophysical Journal), Ghez and her team took into account various effects, such as uncertainties in star positions, ignored by in previous studies. “It’s been a bit like teenagers making emphatic but uninformed statements,” explains Ghez. “In our new paper, we try to take an honest look at where the problems are. We’ve learned that things are more complicated. We’re growing out of our teenage years!”

Besides coming up with a more precise mass measurement, the latest observations refine the distance to the Milky Way’s center: 27,400 light-years, with an uncertainty of 1,300 light-years.

In addition, the group finds no evidence that the central black hole is being gravitationally yanked around by the mass of another. This argues in favor of the team’s higher mass measurement. This new, higher mass value is also more consistent with predictions based on the famous relationship between black-hole mass and the total mass in the spherical component of large galaxies.

Here's the group's paper, their website, and
more imagery.

Before coming to S&T in early June to become editor in chief, I worked at NASA's Goddard Space Flight Center. I wrote press releases and other material to support Goddard missions and scientists. Among my major duties was to promote NASA's next major space observatory: the Gamma-ray Large Area Space Telescope, mission, or GLAST for short. During my time at Goddard I had the pleasure and honor of getting to know some of the mission's leading team members, such as project scientist Steve Ritz, deputy project scientists Neil Gehrels, Julie McEnery, and Dave Thompson, and project manager Kevin Grady.

While I was excited to return to S&T, I felt a sense of letdown to leave Goddard only a few days before GLAST was launched from Cape Canaveral. I missed experiencing firsthand the giddy excitement as team members watched the first data stream down to Earth and seeing their years of labor bear fruit.

I have been following the mission closely since coming to S&T, but I was overjoyed by today’s "coming out party." During an afternoon media telecon, NASA announced a new name for the mission, the Fermi Gamma-ray Space Telescope, and presented first-light science results.

It's going to take me awhile to get used to calling the mission by its new name and acronym FGST, but the choice of the name "Fermi" doesn't surprise me in the slightest. I did not formally submit one of the 12,000 names in NASA's call for public participation, but I mentioned on numerous occasions that it made perfect sense to name the observatory after the great Italian-American physicist Enrico Fermi (1901-1954), who won the 1938 Nobel Prize for physics. Among Fermi’s many enormous contributions to science, he was the first to suggest a viable means for cosmic particles to be accelerated to near-light speeds. This is exactly the kind of thing that GLAST was built to study.

As I expected, no Earth-shattering science results were announced today. After all, FGST has been in space for less than 3 months, and the first few months of all missions are devoted to turning on and checking out the instruments and other spacecraft systems. But we learned today that the two instruments, the Large Area Telescope (LAT) and GLAST Burst Monitor (GBM), are both working as well as anyone could have expected. In just a few days of observing, the LAT, a wide-field instrument that picks up high-energy gamma rays, has already detected all the persistent sources seen by previous missions. The GBM is detecting about one gamma-ray burst per day, slightly higher than the predicted rate.

Now that I sit on the journalist's side of the fence, I can't wait to cover FGST's discoveries in the years ahead. The spacecraft will probably tell us about how active galaxies accelerate particle jets to speeds approaching that of light, why pulsars pulse, how and where cosmic rays are accelerated, how the central engine works in gamma-ray bursts, and how the Sun generates powerful flares. If we get really lucky, FGST might even detect annihilating dark-matter particles and exploding primordial black holes. And since FGST will be observing Mother Nature at her most extreme energies, if we get really, really lucky, FGST might even unveil new laws of physics.

The FGST's journey has just begun. To learn more about this mission, visit the mission website.

Journalists make science look too easy. Plan an experiment, carry it out, get a reportable result. Maybe even an exciting result. Hah.

What we don't see are the countless bomb-outs, meaningless results, equipment tragedies, lost-in-the-noise data, and the wasted years, funding, and careers. If you ever wonder why scientists get wildly enthusiastic when some little thing pays off, it's because they know the alternative.

The stakes are much higher when it comes to investing big budgets in world-class scientific machinery — such as giant astronomical instruments. The professional astronomy community has gotten good in the last half century about evaluating which major projects offer the most promise for the least money, lining up behind these, and letting go of their other fond desires — with a remarkable degree of self-discipline. About every 10 years since the 1960s, American astronomers have produced a "decadal survey" spelling out the future spending projects that seem to offer the best odds.

Planets on the Far Horizon

In 2002, the decadal surveys were joined by a standing Astronomy and Astrophysics Advisory Committee that issues reports on an ongoing basis. Last May it published a report by its Exoplanet Task Force. This report examines the best strategies for discovering, studying, and understanding planets around other stars over the next 5, 10, and 15 years.

In particular, the report looks at the best ways to push the search toward finding small, low-mass planets like Earth, and for understanding "whether our home world is a common or rare outcome of cosmic evolution."

Some of the report's conclusions are no surprise. For instance: refine the precision of planet-hunting spectrographs so they can measure star wobbles as slight as mere centimeters per second. This seems technically doable, and some stars seem to have radial-velocity profiles clean enough that such an engineering investment won't go to waste.

Other recommendations may seem unexpected. For instance, a big, specialized microlensing survey (looking for background stars being temporarily magnified by the gravity of random foreground planets) ought to be so productive (see Sky & Telescope: July 2007, page 18) that a modest, 2-meter telescope should be dedicated to this effort right away.

Just last week, a new argument entered the fray. Planet hunter David Charbonneau (Harvard-Smithsonian Center for Astrophysics) says we could tell a lot about small planets' atmospheres — even finding possible out-of-equilibrium gases that would indicate life — by doing spectral analysis of planet silhouettes transiting their stars. If this is so (it is disputed), it would undercut the need for the long-envisioned, but very expensive, Terrestrial Planet Finder orbiter to image small planets directly. Here's Charbonneau's paper.

A snip from the Exoplanet Task Force report:

"Each of the five different exoplanet finding techniques is most sensitive to planet-star combinations that are very different from the Earth-Sun system. At the same time — for the first time in human history– several different exoplanet discovery techniques are close to finding Earth analogs."

Planet geeks, dive in! Here are the report's abstract and full text (6 MB .pdf file).

Most of us learn that black holes are such potent concentrations of mass and gravity that everything around them gets sucked in, willingly or not. This should especially true near the supermassive black hole, weighing in at some 3½ million Suns, that lurks at the center of our galaxy.

But within the last decade astronomers have come to realize that swarms of stars are buzzing around that cosmic maw in precariously tight orbits. Many of these appear to be quite massive and therefore young, no older than several million years.

Cosmologists have scratched their heads in collective disbelief over this observation. Either these fat stars form elsewhere and somehow migrate inward (improbable, given their youthfulness), or else they condense from great clouds of molecular gas that sweep through the core region. Yet shouldn't the black hole's gravitational force rip the clouds to smithereens as they're dragged to their doom?

Apparently not, based on the results of a new computer simulation that appear in the August 22nd issue of Science. Ian Bonnell (University of St. Andrews) and Ken Rice (University of Edinburgh) followed the evolution of massive gas clouds as they fell into the Milky Way's core. It turns out that some of the gas isn't gobbled up by the supermassive black hole but instead forms an elliptical disk around it — and stars then form in the disk.

Remarkably, many suns end up in orbits no more than 0.1 light-year in radius. And apparently a new burst of star formation occurs whenever a gas cloud dives through the core.

It just goes to show what you can do with more than a year of number-crunching using one of the world's most powerful supercomputers — in this case the Scottish Universities Physics Alliance (SUPA) SGI Altix supercomputer. As Rice notes in an online press release, the key was careful modeling of the gas's heating and cooling during its ordeal. "This tells us how much mass is needed for part of the gas to have enough gravity to overcome its own gas pressure," he says, "and thus form a star."

One of amateur astronomy's most important and rewarding annual rites is Astronomy Day, when hundreds of local clubs provide the public with an introduction to the wonders of stargazing. It's a great opportunity to get an up-close look at — and through — telescopes big and small.

This past year Astronomy Day fell on May 2nd, following a trial autumn event last September. Afterward the Astronomical League judged which events did the best job of fulfilling the goal of "bringing astronomy to the people," and now that the voting is over I'm pleased to announce the winner of this year's S&T Astronomy Day Award.

The envelope, please . . .

It's a tie!

Winner #1 is a consortium of clubs in the Houston area who banded together for a mega-event at George Observatory, located about 30 miles southwest of the city. Home to NASA's Johnson Space Center, Houston is a big, astro-attuned metropolis, so it's little wonder that thousands of people came out for the event last October 20th.

The 6-hour-long bash included too many speakers and events to list, but you can relive the day in words and pictures here. All told, 140 volunteers banded together from the following organizations:

· Astronomical Society of South East Texas
· Fort Bend Astronomy Club
· George Observatory, Houston Museum of Natural Science
· Houston Astronomical Society
· JSC Astronomical Society
· North Houston Astronomy Club

Winner #2 couldn't be more different. The Local Group Astronomy Club, located in Southern California's Santa Clarita Valley, is a small club with big ideas. Its members took a tried-and-true route for their Astronomy Day observance, setting up exhibits and demonstrations in a local library on June 10th. And at day's end they'd spent less than $200 (compared to $4,500 for the celebration at George Observatory).

What set the Local Group apart was the event chairman: 13-year-old Maxwell Ward. According to club president Steve Petzold, Ward is a "prodigious amateur astronomer" who took over much of the planning and coordination of June 10th's activities. He was ably assisted by 12-year-old Christian Borao and, of course, adult members of the club.

Read about the Local Group's activities at the club's website or, better yet, check out its YouTube video about the day's events.

If your club participated in Astronomy Day, please do me two favors. First, add a comment below to let everyone know what you did and how things went. Second, mark your calendar for May 2, 2009, and make sure you're in the running for next year's S&T Astronomy Day Award!

What, exactly, is a "planet"?

The International Astronomical Union thought it had settled the matter when its members voted, two years ago, for a first-ever scientific definition of the term. Boy, were they wrong!

Taken in Prague during the final day of the group's triennial General Assembly, the vote was deemed necessary because an object larger than Pluto (now called Eris) had been discovered in the distant Kuiper Belt. Was it a planet, a giant comet, an asteroid? If Pluto is a planet, then isn't Eris one too? Or if Eris was just the newly crowned "King of the Kuiper Belt," then where did that leave Pluto? Naming rights were at stake!

A "planet," the IAU decided, must circle the Sun (it's not a satellite), has enough mass for gravity to have drawn it into a round shape, and has enough mass to have cleared out everything else in its orbital neighborhood through impact or scattering. This left Pluto and Eris, literally and formally, out in the cold.

Few planetary scientists like the IAU's definition, which is confusing and vague. Within days of the IAU's vote, some of them started a recall petition.

"Clearing" is dynamicist-speak for a gravitationally dominant object, such as Jupiter. Except that Jupiter has thousands of asteroids, called Trojans, that share its orbit — so maybe Jupiter and, likewise, Saturn aren't planets. Earth still has to fend off stray asteroids that pass its way, so is Earth a planet? And if Earth were out at the orbit of Neptune, it wouldn't have the gravitational chops to dominate much of anything. No clearing, no planethood.

In its haste to get something on the books, the IAU failed to define what the maximum size for a planet should be, or to deal with the growing count of planets known to circle other stars.

Oh, and did I mention that the IAU also decided to make Pluto, Eris, and Ceres charter members of a new class of "dwarf planets" that aren't technically planets? To add to the confusion, two months ago the IAU made good on a action item left over from Prague that Pluto and bodies like it would henceforth be called Plutoids.

To try to bring some sanity to this mess, Pluto petitioners Alan Stern (Southwest Research Institute) and Mark Sykes (Planetary Science Institute) felt that it would be in science's best interest to hold a meeting to explore what "planet" really means. That "Great Planet Debate" is under way at Johns Hopkins University's Applied Physics Laboratory in Laurel, Maryland.

Although it's been billed as a scientific meeting, the mix of 150 attendees is skewed toward educators, students, and reporters interested in the outcome. Still, the few dozen scientists here have had plenty of give and take. Some attendees, like Hal Levison (Southwest Research Institute) maintain that the IAU basically got it right, that gravitational dominance is the single best truth-test for planethood.

But others (and probably most others, I sense), think "roundness" is a better metric. As Sykes points out, an object becomes round once its own gravity wins out over the material strength of whatever it's made of. Geology happens. But how round is "round"? Where do you draw the line between "round enough" and "a little too bumpy"?

Capping off the first day's activities was a debate of sorts between Sykes and Neil deGrasse Tyson, who directs the Hayden Planetarium in New York City. Tyson's planetary preferences became clear when the Rose Center opened in 2000. Pluto was conspicuously missing from its "Walk of the Planets."

Ira Flatow, host of National Public Radio's "Talk of the Nation," gamely tried to moderate the discussion. But he could do little to contain the flamboyant and sometimes animated verbal sparring that ensued.

Sykes, a "roundness" devotee, argued that the IAU should have been more, not less, inclusive in its definition. By his count, the solar system now boasts 13 bodies that qualify as planets: the usual eight plus Ceres, Eris, Pluto, and Charon (which he claims isn't really Pluto's satellite because their combined center of mass lies between them).

But Tyson countered that "The word planet has lost all scientific value." Instead, he said, come up with a lexicon that does a better job of characterizing the objects — define it however you want, he stressed, but make it useful.

At least they agree that science shouldn't be legislated by a vote. It's an often-messy enterprise whose outcome often falls outside neat categorizations. And that's part of what makes science so exciting.

For a few seconds on Monday (Aug. 11, 2008) the diehard Cassini spacecraft skimmed only 50 kilometers (30 miles) above the surface of Saturn's little moon Enceladus, which leaped to the forefront of solar-system studies when Cassini discovered active ice geysers spraying from it in 2005. The data are streaming back, and NASA is posting the preliminary raw images.

"Cassini focused its cameras and other remote sensing instruments on Enceladus with an emphasis on the moon's south pole" says a NASA press release, "where parallel stripes or fissures dubbed 'tiger stripes' line the region. That area is of particular interest because geysers of water-ice and vapor jet out of the fissures and supply material to Saturn's E ring."

Check the NASA Cassini site for the latest. And see imaging team leader Carolyn Porco's blog post.

"Two more Enceladus flybys are planned for October," notes the NASA release. "The first of those will cut Monday's flyby distance in half and bring the spacecraft to a remarkable 25 kilometers (16 miles) from the surface." A resolution of 3.7 meters per pixel should be achieved.

Every year I look forward to representing Sky & Telescope at the annual meeting of the Astronomical League. Now boasting 15,000 members and 270 participating clubs, the League serves as the principal voice for amateur astronomy in the United States.

S&T and the League go way back — in fact, Charlie Federer, the magazine's founder, helped launch the organization in 1941 and coauthored its first constitution. Since then we've joined forces often to grow the ranks of stargazers, with special emphasis on encouraging "newbies" to get involved.

The League's youth awards are always featured at its annual convention, and with good reason. Young skygazers enrich our activities at every level. They depend on us to guide them in their explorations of the heavens, just as we'll need them to sustain our hobby in the decades ahead. So at this year's festivities, the League honored several budding astronomers whose astronomical talents and enthusiasm really stand out.

John Hodge II received the National Young Astronomer Award for 2008. A senior at Spring Valley High School in Columbia, South Carolina, Hodge took up the challenge of observing cataclysmic variable stars (compact binaries that periodically erupt after the white-dwarf primary accumulates matter from its companion) and deriving a detailed light curve for IP Pegasi in particular. I had a nice chat with John and his proud family, who'd come to Des Moines for the presentation.

In the past, the top NYAA award-winner got to cart home a Meade 10-inch LX200 telescope — but not this year, because the company abruptly decided to end its long-running support of the program. Fortunately, Scott Roberts (a former Meade executive) stepped forward to provide young Hodge with a 5-inch apochromatic refractor from his new company, Explore Scientific.

Runners up for this year's NYAA award are Lara Knorek, a senior at Kalamazoo Area Math and Science Center in Kalamazoo, Michigan, and Neil Pearson, who'll be graduating from Clear Creek High School in Evergreen, Colorado. She analyzed light curves from supernovae, and he is an accomplished science student who has built reflecting telescopes from scratch.

Carroll Iorg, the League's vice president and NYAA coordinator, encourages club officers to keep an eye out for candidates for the award. "Whether building an astronomy-related business, completing a science project related to astronomy, or doing research at an summer astronomy camp," Iorg notes, "these types of projects have a good chance of being top finishers."

The League's other major recognition of youth involvement is the Jack Horkheimer Award for exceptional service by a young astronomer. Since 1998, public television's gregarious late-night stargazer has provided a $1,000 prize to 18-and-under enthusiasts who've devoted themselves to helping their clubs.

This year's Horkheimer winner is Christina Lee, who's still attending Central Catholic High School in Portland, Oregon, but already boasts an impressive astro-résumé. Lee has been an active member of Portland's Rose City Astronomers and also shows off the night sky with the Vancouver Sidewalk Astronomers. Meanwhile, she participates in the Stardust@home program (scanning images for dust particles captured by the spacecraft), and she's analyzed and classified more than 1,000 galaxy images taken by the Sloan Digital Sky Survey.

Neil Pearson took second prize in the 2008 Horkheimer competition. He's been a student member of the Denver Astronomical Society for eight years, worked with other club members to maintain and operate Chamberlin Observatory's 20-inch refractor, and has ground, polished and figured his own 8-inch f/6 telescope mirror. Third prize went to Mark Sutter, an active member of the Des Moines Astronomical Society (which hosted the conference).

Hearty congratulations to this year's winners: John, Christina, Lara, Neil, and Mark! Will some youngster you know be taking home an award next year?

For any of you contemplating a career in planetary science, let me take a cue from the classic 1967 movie The Graduate and offer one single word of advice: "Modeling."

Trying to figure out how planets form and what happens after they do is arguably one of the hottest research areas in astronomy. With the count of known extrasolar planets now topping 300, a tenth of those being multi-planet systems, it's becoming clear that there's a whole smorgasbord of planetary combinations out there — few of which look anything like ours — and we don't know why.

One problem is that dynamicists can't yet harness enough computing horsepower to tackle all the processes that take place in a planet-forming disk: how long the disk's gas hangs around, how and where planet embryos form, and what kind of collisional chaos ensues. To date, most planet-building computations have sidestepped all the messy intricacies of how gas and solid bodies interact within the disk. Theorists simply hit "go" once the gas has dissipated.

But recently a trio of researchers at Northwestern University in Illinois followed the birth of more than 100 hypothetical solar systems from beginning to end. Each simulation ran for 500 million years — long enough for the star-encircling disks to spawn and then for the young worlds to interact with the disk and duke it out until only a few remain.

Writing in August 8th's Science, Edward Thommes, Soko Matsumura, and Frederic Rasio determined that exoplanetary roulette depends on how much mass the disk has to work with, and whether the disk sticks around long enough for giant planets to form. At one extreme, wimpy disks that dissipate quickly will produce no gas-giant planets at all. But massive, longer-lasting disks end up with multiple giants that jostle each other gravitationally.

The new simulations even yield the "hot Jupiters" that observers have unexpectedly discovered — giant planets that migrated inward but stopped just short of being swallowed by their host stars either when they reached the disk's inner edge or the disk itself dissipated.

The real take-home message is that our particular planetary mix — with several little rocky ones and several more big gassy ones, all coexisting peacefully in nearly circular orbits — is the exception, not the rule.

"The solar system had to be born under just the right conditions to become this quiet place we see," notes Rasio in a Northwestern press release. "The vast majority of other planetary systems didn't have these special properties at birth and became something very different."

As I prepare to end my summer internship at Sky & Telescope, I find it ironic that the last two stories I write both concern the "dark" unknowns of astronomy. Dark matter, like dark energy, is something that astronomers can't seem to escape. Some argue that that's only because we don't want to — we like having a crutch to lean on, if you will — but this opinion hasn't trickled down to the textbooks I've used.

A survey by an international team of astronomers recently released results that once again bring dark matter into the discussion. Using a new approach to weigh galaxies, the researchers explored the relationship between mass and luminosity for medium- to high-mass elliptical galaxies. Their study of 70 galaxies using the Hubble Space Telescope's Advanced Camera for Survey (ACS), in conjunction with information from the Sloan Digital Sky Survey, confirms that for progressively larger elliptical galaxies, their masses do indeed increase more rapidly than their luminosities. This discrepancy raises the possibility of a higher fraction of dark matter to regular matter in larger galaxies.

Scientists normally estimate the masses of distant elliptical galaxies by measuring their sizes and the velocities of the stars within them, explains team member Adam Bolton (University of Hawaii). With these quantities and a little math they calculate the dynamical mass, which isn't the true mass but does relate to it — exactly how, though, depends on things like the stars' orbits, the distribution of material throughout the galaxy, and the ratio between stars and dark matter.

For about 20 years astronomers have known that one elliptical galaxy twice as massive as another will be significantly less than twice as luminous, Bolton says. Scientists have tossed around two main explanations for why. On the one hand, they may not have accounted for something in the details relating dynamical and actual masses. On the other, there could be some sort of systematic increase in how much mass larger galaxies have compared to how much light they emit.

Instead of calculating dynamical masses, Bolton and his colleagues used gravitational lenses to estimate the galaxies' true masses. These lenses occur because mass bends the space-time around it. When an enormous object — like an elliptical galaxy — lies between us and a more distant galaxy, the light coming from the background galaxy travels through the space-time dip that the nearer object creates. The dip acts like a lens, redirecting the light. Often, multiple images of the farther object appear in an arc around the lens, and can be up to 30 times brighter than the distant galaxy's original image.

The astronomers used the lens images, as well as the distances to each galaxy, to calculate the nearer galaxies' masses. Their true-mass measurements, combined with previous dynamical measurements, show that the relationship between dynamical and actual masses does not vary like the mass and luminosity one does. So the higher mass-to-light ratio appears to be the right answer.

There's still much to debate, Bolton cautions. The results don't explain what the extra mass is. It could be that there's a larger fraction of dark matter to regular matter in these galaxies. The stars themselves might also have a higher mass-to-light ratio.

"The consensus based upon modeling of stellar populations and simulations of galaxy evolution seems to favor the dark-matter explanation," Bolton says. "But for those with a deeply held objection to the entire concept of 'dark matter,' the stellar-mass effect will of course seem more plausible."

The team did see a slower decline in mass density than light density as they looked farther out from the galaxies' centers, Bolton adds. This slower decline requires some form of unseen matter. The other explanation — that the stars themselves have a different mass-to-light ratio based on their position within the galaxy — Bolton finds implausible.

The results prove the applicability of the lens-weighing method and open the door for "other people to do clever science that hasn't occurred to us," Bolton concludes.

For the most part, scientists have come to terms with the existence of an unknown antigravity force permeating the cosmos. This "dark energy" — a conveniently ambiguous term for something no one understands — sticks its nose into cosmology on a regular basis and, increasingly, won't be denied.

While we're nowhere near cracking dark energy's secrets, a team of astronomers from the University of Hawaii's Institute for Astronomy has confirmed its effects on the microwave background radiation we see from the early universe. The team's data also confirm theories that large-scale cosmic structures — shaped in part by dark energy — should give rise to anomalies in this radiation.

The astronomers, led by István Szapudi, looked for what's called the late-time integrated Sachs-Wolfe (ISW) effect. It's a lot of words to describe something relatively straightforward:

Imagine a rubber sheet stretched taut. If you take, say, five dinner plates and set them close to each other on the sheet, they create a deep valley. If instead you spread the plates farther out on the sheet, they'll make a shallower valley.

Now add the astronomy: the plates are the galaxies of a gigantic supercluster 500 million light-years across. The sheet is space-time, and the galaxies in it move apart from each other because space-time is expanding like stretched rubber. (That's what astronomers mean by "expansion of the universe.") Dark energy speeds up the rate of this expansion.

A photon from the far background travels toward you though space-time like a marble rolling on the sheet. It falls down one side of the supercluster's valley, thereby gaining a little energy. In a non-expanding universe, the photon would use up that same amount of energy when it climbed the opposite side, with no net effect.

But in an expanding universe, space-time stretches and the supercluster's valley flattens out during the photon's 500-million-year journey across the valley. When the photo arrives at the other side, the hill it climbs up is shorter than the hill it first went down. So the photon keeps some of the energy that it gained when falling in. This difference appears as a temperature increase — in this case, a change of ninety millionths of one kelvin (i.e. really really small).

On the other hand, if the photon first climbed up a hill — a region with a below-average number of galaxies such as a supervoid — that hill would be lower by the time the photon came back down. The photon would never regain all the energy it lost by climbing. In this case, the photon would be slightly colder.

That's the late-time integrated Sachs-Wolfe effect.

The Hawaii team studied this effect on microwaves that passed through 50 superclusters and 50 supervoids mapped at various places on the sky by the Sloan Digital Sky Survey. The microwaves come from the cosmic microwave background (CMB) radiation — the blotched-looking image at right that is our earliest picture of the universe, originating when matter and light separated a mere 380,000 years after the Big Bang.

Because temperature fluctuations existed in the CMB even before the radiation passed through later superstructures, the astronomers had to find a way to reveal the ISW effect hiding in this "noise." They did so by stacking CMB images of the sky that correspond to superstructures' locations.

"Each time you add another image to the stack, the CMB fluctuations average out, thus get smaller, and our desired ISW signal gets stronger," explains Szapudi. Summing up the stacks, the scientists found that slightly warm and cool spots on the microwave background indeed line up with superclusters and supervoids, respectively. The spots' sizes and strengths across cosmic ages match what accelerating expansion predicts.

Scientists have studied the ISW effect before, and the Hawaii group's results bring us no closer to understanding dark energy's nature, says Mario Livio (Space Telescope Science Institute). Still, the study supports other teams' work, particularly theories that the prominent "Cold Spot" — a (you guessed it) very cold region on the CMB discovered in 2004 — results from a supervoid (still unconfirmed, but more likely now). And the further evidence for dark energy's existence may be a solid step toward constraining current cosmological models, notes Sean Carroll (Caltech).

The paper, lead-authored by Benjamin Granett in collaboration with Szapudi and Mark Neyrinck, will appear in a future issue of the Astrophysical Journal Letters.

More information is in an Institute for Astronomy press release, along with some great images and animations.

Eta Carinae is one of the weirdest stars in the Milky Way. It shines with 5 million times the Sun’s luminosity and spits out the equivalent of Jupiter’s mass in its stellar wind each year. (The Sun would unravel itself in a millennium if it spewed out so much stuff.) It’s also one of the galaxy’s most massive stars at roughly 100 solar masses — even after its “Great Eruption” in the 1840s, in which it ejected at least 10 times the Sun’s mass into space.

There’s something strange going on with the light that travels 7,500 light-years to us from Eta Car, too. Changes in the star’s spectra are heralding a significant event in January 2009, which may settle once and for all whether Eta Car has a massive, unseen stellar companion.

Eta Car’s spectra undergoes a 5½-year cycle. Since Augusto Damineli (University of São Paolo, Brazil) first discovered it more than a decade ago, scientists have argued over why this regular period exists. Most now favor an undetected stellar binary companion as the instigator — the key word there being undetected. Astronomers have never managed to find it because the Homunculus Nebula surrounding Eta Car prevents good observations and Eta Car itself is so bright that it blinds instruments to anything nearby.

Equally peculiar is that the star’s X-ray emissions abruptly disappear for a few months during every cycle. This drop happens with a disturbing regularity, recurring as predicted in 1997 and 2003. As the next “X-ray crash” approaches this January, a team led by Kris Davidson (University of Minnesota) has confirmed an unusually intense line in Eta Car’s spectra using the 8-meter Gemini South telescope in Chile. The scientists suggest that the feature supports the binary-companion theory and also may indicate additional mass ejection from the star.

Specifically, the astronomers detected an increased intensity in a line they think occurs when a helium atom has first lost both of its two electrons and then regains one. There are multiple energy levels within an atom at which an electron can exist, and as this captured electron falls down to a lower energy level from a higher one it emits light with a very particular wavelength, which scientists measure as a spectral line.

Often these “recombination lines” appear in spectra from stars with very hot, strong stellar winds. Yet for many years, Eta Car observers didn’t see He II features (He II is helium missing an electron). When they finally did spot a He II line in 2003, the line was strongest around the time of the X-ray drop, says team member John Martin (University of Illinois at Springfield) — and is the exact same line the astronomers see now.

Scientists still don’t know how much of Eta Car’s weirdness results from the star’s intrinsic properties and how much from the influence of its presumed companion. The X rays themselves may arise from a shock front created when Eta Car’s wind collides with that of its unseen companion. As it travels around the primary in a highly elongated orbit, the companion star’s wind carves out a three-dimensional version of a speedboat’s bow shock, explains Michael Corcoran (NASA/Goddard). The high energies produced in this collision suggest that both winds are hot and dense and that the companion is a massive, hydrogen-burning O star — although nowhere near as heavy as Eta Car.

So why do these X rays periodically disappear? Nathan Smith (University of California, Berkeley) suggests that as the companion swings close by Eta Car, it disrupts the primary’s stellar wind. This event throws Eta Car’s outflow into chaos, and it takes a few months to rebuild itself.

Another possibility is that the thick, slower wind coming off Eta Car’s equatorial region engulfs the companion star at its closest approach, says Corcoran. It’s rather like how the bow shock bends around the speedboat when it makes a fast turn.

But the strong He II emission hints that there’s something else going on besides the winds’ crash, Martin says. “In order to get this kind of He II line, you need to ionize helium twice, which takes a lot of energy.” And although X rays are highly energetic, Martin and his colleagues don’t think the winds’ collision emits enough energy to completely explain the spectral features.

Martin suggests that higher energy levels could arise if Eta Car throws off a lot of material from its equator as the companion approaches. This material would impact the shock front, boosting the clash’s effect. And indeed you would expect a rapidly spinning object to fling material off at its equator, says Smith. But it remains unknown whether the companion’s close passes have some tidal influence on Eta Car’s abnormally high rotation speed.

Eta Car also tantalizes those who model star formation in the early universe, since scientists think that such enormous stars peppered space at that time. Naoki Yoshida (Nagoya University, Japan) and Lars Hernquist (Harvard-Smithsonian Center for Astrophysics) recently presented results from computer models suggesting that stars 100 times the Sun’s mass appeared early after the Big Bang and formed relatively easily. As few stars currently exist on such a scale (compared to the number of less massive stars), observations of Eta Car may elucidate the hows and whys of those first giants. The team’s paper appears in last Friday’s Science.

If Eta Car behaves in January 2009 like it has in the past, dissidents from the binary theory will probably have to surrender. Yet the star could also surprise everyone. The companion might trigger another Great Eruption, for example — we still don’t know if it set off the last one. And with such a high mass, Eta Car may go supernova in the near future. If it does, it may deprive scientists of solving its mystery forever.

As the second-largest moon in the solar system, Saturn's Titan already boasts a nitrogen-rich atmosphere, dynamic weather that sometimes triggers thunderstorms, mountain vistas, and vast sand seas.

Now you can add lake-front property.

After circling Saturn for years, the Cassini spacecraft finally has solid evidence that a large, flat area near Titan's south pole is almost certainly liquid ethane. This hydrocarbon-filled lake, nicknamed Ontario Lacus by the mission's scientists, covers roughly 7,800 square miles (20,000 square km), slightly larger than Lake Ontario in North America. It's tantalized the Cassini team ever since the spacecraft's main camera discovered dark polar patches in 2005 while peering through the dense, haze-choked atmosphere using an infrared filter.

Follow-up scans with an onboard radar showed the patches to be dark at radar wavelengths as well — just the sort of signature you'd expect from a fluid pool. But the team had to be cautious about calling it a lake outright because other surfaces could conceivably (though improbably) mimic the visible and radar signatures.

The lake hypothesis reached its splash point last December, when Cassini's visible and infrared mapping spectrometer got a good look at the area during one of several dozen flybys of Titan to date. VIMS analyzed the surface's infrared reflectivity between 2 and 5 microns, using wavelengths at which the atmosphere is transparent. A handful of absorptions in the spectra match the ones expected for liquid ethane. Details of the detective work appear in the July 31st issue of Nature.


Interplanetary chemists once imagined the surface of Titan to be completely inundated by a hydrocarbon sea. That's because sunlight causes methane gas in the moon's atmosphere to break down and recombine as ethane (along with more complex hydrocarbons). Cassini dismissed the idea of a global ocean soon after arriving, but the Huygens probe that it dropped onto Titan's surface photographed river systems as it descended and plopped onto moist ground.

Over the next two years of its historic Saturn-circling mission, Cassini is expected to make at least two dozen more flybys of Titan. Mission scientists hope to use those passes to map out the full extent of the lake regions and gain a more complete geologic picture of this strange world.

Today marks the 50th anniversary of the foundation of NASA. In 1958 the U.S. Congress established the National Aeronautics and Space Administration by transforming the four-decade-old National Advisory Committee for Aeronautics (NACA). The new agency didn't begin operations for two more months, however. So, look for another anniversary milestone on October 1st.

With NASA at 50, you're bound to see all manner of remembrances. During just the past few months, I've seen several TV series and documentaries about the space program.

Last week, NASA announced another birthday-worthy resource: a searchable archive of space photography at NASA Images. This is a cooperative effort with NASA and the Internet Archive, where you can find all manner of interesting stuff, including old movies and the famed "Wayback Machine" that will show you what many websites looked like oh, so long ago.

After receiving the press release last Thursday, I eagerly went to the site, but didn't get much further than the opening screen. The following days were better, but I still run into slowness. As I was writing this, I got the error message: "NASA Images is experiencing high load, please wait 30 seconds and reload."

The site is a bit different than the NASA Planetary Photojournal or other image repositories. NASA Images is set up like "light box" software, in which you search through the database using keywords to display thumbnails images, and then you save them to your "workspace." This is the electronic version of laying out slides on a light box. With thousands of images to pore over, it's a good way of keeping track of the images you might be interested in without losing track of some of them. And it's not just photos; there are movies too.

Of the images I looked at and downloaded, the pictures were of decent size for computer-display use (about a megapixel). So they would be great for a school project, newsletters, or a PowerPoint presentation, but barely enough resolution for, say, publishing very large in Sky & Telescope.

The site offers ways of embedding the images into a website of your own and otherwise sharing images you find with friends, but I didn't attempt either of these features.

There's lots to look through at the site — and all public domain! Happy hunting.

It's a calm summer evening here in Boston, but high above me in space an unseen battle rages.

Thousands of miles up, an invisible wind of electrified gas and magnetic fields from the Sun constantly slams into and around Earth's magnetosphere, the protective bubble created by our planet's magnetic field. Meanwhile, inside the bubble, a barrage of charged particles zip up and down along field lines, creating powerful electric currents and a dangerous radiation environment. And that's on a quiet night.

But when the solar wind gets whipped up, Earth's defenses start to break down. Waves of solar-wind plasma leak into our electromagnetic cocoon. Choked with extra mass and field lines, the night-side magnetosphere sometimes explodes with a violent release of pent-up energy — termed a substorm — that causes a sudden brightening and poleward spreading of auroras in the upper atmosphere.

Space physicists have debated the cause of substorms for decades. Some thought the trigger involved a disruption of powerful electric currents (think "humongous short circuit") about 40,000 miles down the magnetosphere's tail. Others believed the source region to be two or three times farther out, where magnetic field lines become pinched together, reconnect, and snap inward — like suddenly letting go of stretched rubber bands.

Here's the chicken-and-egg conundrum: Current disruptions should trigger magnetic reconnections, but likewise magnetic reconnections should cause current disruptions. In fact, measurements made in 2005 by a European-Chinese space collaboration showed that sometimes the two phenomena happen at the same time and in roughly the same place.

To pin down which is the cause and which the effect, last year NASA launched a quintet of identical sat